In a pot over medium heat, heat oil. Add onions, garlic, and ginger. Cook, stirring regularly, until aromatic. Add chicken and cook, turning occasionally, until juices run clear. Add tomatoes and cook, mashing with until softened.
Add fish sauce and continue to cook, stirring occasionally, for about 2 to 3 minutes. Add water. Lower heat, cover, and simmer until chicken is cooked through.
Add long beans and pickled tamarind leaves, and continue to cook until long beans are tender yet crisp. Add tamarind base powder, stirring to dissolve. Season with salt to taste. Serve hot.